Hydride vapor-phase epitaxy (HVPE) is an important technique for the epitaxial growth of various semiconductors, such as gallium nitride, an important technological material. For example, GaN is currently used in the manufacture of blue light emitting diodes, semiconductor lasers, and other opto-electronic devices. The background of the related art will be discussed with particular reference to the deposition of GaN, although all of the discussion can be applied to several other compounds such as GaAs.
HVPE is a currently favored technique for GaN buffer layer deposition, because it provides relatively rapid growth in a cost-effective manner. It is also favored in producing thick semiconductor nonlinear optical material, such as GaAs, because of the high growth rates and low background impurity levels. In HVPE, growth of GaN or GaAs proceeds due to the high temperature, vapor-phase reaction between gallium monochloride (GaCl) and either ammonia (NH3) or arsine (AsH3). The ammonia or arsine is supplied from a standard gas source, while the GaCl is produced by passing HCl over a heated liquid gallium supply. The two gases (ammonia and GaCl for GaN) are directed towards a heated substrate where they react to produce solid GaN on the substrate surface.
While HVPE allows for high growth rates of GaN, there are certain difficulties associated with HVPE as a technique that have made it unattractive for industrial applications. First, in current HVPE processes, the metallic source is positioned within the growth chamber in a horizontal configuration. This is done because the GaCl produced in the reaction must be maintained at high temperature (greater than 400° C.) to prevent its decomposition to the low-temperature stable phase of gallium trichloride (GaCl3.) GaCl3 cannot be used for HVPE because the presence of the two extra chlorine atoms pushes the growth reaction towards etching rather than deposition. Prior to this invention, the only practical method for maintaining GaCl temperature was to have the metallic source directly inside the growth chamber, which is always maintained at an elevated temperature for the crystal growth process. A multiple-zone furnace may be used, with a substrate zone and GaCl production zone, with each set to an optimal temperature for the respective process. In all cases however, the GaCl production zone and substrate are contained within the same enclosure. As a result of this, it is necessary to disturb the entire enclosure to service (refill) the metallic source as it becomes depleted with use.
A second limitation to the industrial use of HVPE is the parasitic growth chamber wall deposits. A consequence of the HVPE deposition reaction is that it is thermodynamically more favorable at lower temperatures. To inhibit wall deposits, the reactor environment is maintained in such a manner that the substrate is cooler than the reactor walls. Typically this is accomplished by heating the substrate with a furnace or heater which is external to the growth chamber. Even so, the deposition rate is not nearly as selective as in other industrial growth processes such as organo-metallic vapor-phase epitaxy, and deposition still does occur on the growth chamber walls. Once deposition has nucleated on such surfaces the reaction rates increase leading to a nonuniform and decreasing deposition rate at the substrate. Additionally, wall deposits may flake off or shed particles that are carried downstream to the substrate zone, where they can become incorporated into the growing crystal as second-phase defects.
A third limitation is the mixing of group III and group V reactant compounds such as GaCl and ammonia before the preferred deposition region at the substrate. Because of the reactant chemistry and the near uniform heating throughout the growth chamber the gas species can react before the substrate forming gas-phase borne particulates, which may be incorporated into the crystal growth at the substrate forming second-phase defects in the crystal.